Cryogenic magnet systems are used in many of the presently available and proposed MRI devices. Liquid helium is used to cool the electromagnets to the low superconducting temperature required. One of the problems with liquid helium cooled systems is that low temperature is maintained by the "Boil Off" of the liquid helium that occurs when its temperature reaches 4.2.degree. Kelvin (at atmospheric pressure). Due to the "Boil Off" the helium has to be replaced periodically.
To reduce the boil off of expensive helium, MRI magnet systems typically contain a heat radiation shielding means which itself is usually cooled by a nitrogen system. In most cases the nitrogen, like the helium, is supplied in bottles and is boiled off in the process of cooling the shield. The problem to be solved by this invention is to reduce the cost and effort associated with cooling the shield.